Fixes two bugs that appeared since the introduction of the struct:
- when reading permissions from the journal, null ("") was read as
empty-not-null
- when reading permissinos from the server, empty ("") was read as null
Addresses #4608
Add a test to test the data fingerprint feature make me realize it was broken.
The code was relying in the distinction between empty and null QByteArray,
but this was a bad idea as this difference is lost when going through QString.
